Raise an error if no unprocessed payments.

Fixes #5436

/cc @athal7 This breaks an exchanges rake task spec spec.
I spent a bunch of time trying to make it work right,
but haven't figure out proper way to do it.  It appears the while order.next
loop completes order because of default credit card bug.  Basically need a solution
to: Refs #5327
added code to shipping methods

better layout

place shipping_method.code on selected shipping rate

Fixes #5439
better way to redefine existing methods: allows to call original behaviour with "super"
Fixes #5358
War on class_eval. refs #5358
Update mail settings configuration change.

Prevents issues getting opened up all over the place about it.
Fixes #5367
Deleted the continue button from adjustment index

On the adjustment index (in the admin) was a continue button.
The button only shows up on this page, it's a leftover from the old 'manual add an order' proces in the admin.

I deleted it.

Fixes #5395
Regression spec for 'stack level too deep' #5384

Fixes #5384
Mention we accept PRs for documentation.
Add in fix from @andrzejpolis Fixes #5384 Fixes #5386
Go to cart and then call next, to trigger hooks
Fixes #5216
Fix specs broken by e6865c6
Fix guides syntax.